The present invention deals with an optical system. More specifically, the present invention deals with a fingerprint imaging system.
Fingerprint imaging systems implementing prism technology have been used in the past. Such systems typically take advantage of the total internal reflection characteristics of a prism. In such systems, a light (which is conventionally emitted from a point source through a collimating lens) is input through one face of the prism. The fingerprint is imaged off another face of the prism and the image is reflected through an output face of the prism.
In such prior systems, a right angle prism is typically used. Thus, the light source is provided through one leg of the prism and the fingerprint is imaged off of the hypotenuse of the prism (i.e., the hypotenuse is used as the reflecting surface of the prism). The image is then reflected out of the other leg of the prism.
The optical image travels through a corrective lens and is provided to an imaging device such as a linear charge coupled device or an area array charge coupled device. The intensity of the light provided to the charge coupled device is converted to an analog signal which is used in a variety of ways.
Such prior systems are plagued by a number of problems. First, it is very difficult to completely preclude external light from entering through the reflecting face of the prism. Due to the total internal reflection characteristics of the prism, once external light has entered the prism it acts essentially as noise which causes a loss of image contrast and therefore loss of definition of the image reflected out through the output face of the prism. Also, external light can tend to invert the image (i.e., either invert it from black to white or white to black).
Another significant problem associated with prior fingerprint imaging systems is caused by debris or residue which is left on the prism face. If the residue is particulate matter or other light obstructing matter, it can result in an erroneously scanned image and therefore an erroneous image at the output of the prism. However, residue can also exist in the form of excess moisture or oil which is left on the reflecting face. This also has a deleterious effect on the image reflecting capability of the prism.
the problems which result from both external light entering the prism, and debris or residue accumulating on the prism face are typically referred to as ghosting.
Image distortion is another problem associated with prior imaging systems. Image distortion results in a skewed image (for example, where the image is a circle, it can be reflected out of the prism as an elliptical shape). The distortion can arise from aberrations introduced by the optics in the system. Prior systems have had difficulty in efficiently and economically adjusting the image reflected out of the prism to accommodate for such distortion.
Yet another problem associated with prior imaging systems is the inability to obtain different or optimum image sizes. The lens which receives the reflected image from the prism in prior systems is typically a correcting lens which does not provide a flat field image to the imaging device. Therefore, the correction provided by the lens is only effective for one predetermined distance between the corrective lens and the imaging device. It is thus impossible to change the size of the image provided to the imaging device by moving the corrective lens relative to the imager without destroying the corrective effect provided by the correcting lens.